Highly pure para-methyl styrene has recently become significant as a monomer or comonomer for making specialty vinyltoluene polymers and copolymers having improved high temperature properties, e.g., Vicat softening point and density, compared to polystyrene. Para-methyl styrene can be produced commercially by catalytic dehydrogenation of para-ethyltoluene.
In numerous processes described in the patent literature, for example U.S. Pat. Nos. 3,626,020 to Neuzil, 3,663,638 to Neuzil, 3,665,046 to deRosset, 3,668,266 to Chen et al., 3,686,342 to Neuzil et al., 3,700,744 to Berger et al., 3,734,974 to Neuzil, 3,894,109 to Rosback, 3,997,620 to Neuzil, 4,482,777 to Neuzil, and 4,014,949 to Hedge, particular zeolitic adsorbents are used to separate the para isomer of dialkyl substituted monocyclic aromatics from the other isomers, particularly para-xylene from other xylene isomers. Other patents, e.g., U.S. Pat. Nos. 4,051,192 to Neuzil et al and 4,423,279 to Kulprathipanja specifically disclose adsorptive separations selective for the para-isomer of ethyltoluene with potassium or barium-exchanged X zeolites or a pyrolyzed zeolitic adsorben, respectively. Many of the above patents use benzene, toluene, or p-diethylbenzene as the desorbent. P-diethylbenzene (p-DEB) has become the commercial standard for para-xylene separation. However, p-DEB suffers in the process for separating feed mixtures containing C.sub.9 aromatic because the boiling point of p-DEB is too close to the boiling point of C.sub.9 aromatics in the feed. Because the C.sub.9 aromatics are difficult to separate from p-DEB by simple fractionation, the C.sub.9 aromatics would gradually build up in the desorbent, which must be recycled for economic reasons. U.S. Pat. No. 3,686,342, supra, mentions tetralin as a possible heavy desorbent for the paraxylene separation process, but does not address the problem that the preferred desorbents may have in separating feeds containing C.sub.9 aromatics. Therefore, a higher boiling point material that meets the selectivity requirements for desorbents and can be separated from C.sub.9 aromatics is desirable.
It is also known that crystalline aluminosilicates or zeolites are used in adsorption separations of various mixtures in the form of agglomerates having high physical strength and attrition resistance. Methods for forming the crystalline powders into such agglomerates include the addition of an inorganic binder, generally a clay comprising a silicon dioxide and aluminum oxide, to the high purity zeolite powder in wet mixture. The blended clay zeolite mixture is extruded into cylindrical type pellets or formed into beads which are subsequently calcined in order to convert the clay to an amorphous binder of considerable mechanical strength. As binders, clays of the kaolin type, water permeable organic polymers or silica are generally used.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Pat. No. 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRosset U.S. Pat. No. 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour.
The invention may also be practiced in a cocurrent, pulsed batch process, like that described in U.S. Pat. No. 4,159,284 or in a cocurrent pulsed continuous process, like that disclosed in Gerhold, U.S. Pat. Nos. 4,402,832 and 4,478,721.
The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well-known, but for reference thereto, Zinnen et al. U.S. Pat. No. 4,642,397 is incorporated herein.
I have discovered a process for employing a zeolite adsorbent for the separation of p-ethyltoluene from other C.sub.9 and C.sub.8 aromatic hydrocarbons and, particularly, a desorbent which is a substantial improvement in a process for separating p-ethyltoluene from feed mixtures containing other C.sub.9 aromatic impurities, including isomers of p-ethyltoluene.